A variety of coating compositions is used for the painting of automobiles, including lacquers and enamels. In general, lacquers have relatively high percentages of solvent and contain high molecular weight polymers. By contrast, enamels are based on low molecular weight polymers which require lower concentrations of solvent. In order to achieve equivalent protection using enamels, the molecular weight of the polymers used in enamels is increased through crosslinking. Typically, acid catalyzed reactions are used for crosslinking, with melamine formaldehyde resins as crosslinking agents, in combination with elevated temperatures, for example, of about 250.degree. F.
Enamels based on acrylic polymers are particularly desirable in automotive applications, since these enamels are especially resistant to gasoline and brake fluid. However, the use of melamine formaldehyde crosslinking agents typically results in the emission of alcohols or formaldehyde from the crosslinking reaction combined with a significant weight loss that causes shrinkage of the films made from the enamels. Accordingly, effort has been directed toward the development of improved crosslinking systems which avoid these potential difficulties in the curing reaction and in the final crosslinked product.